This invention relates to solar water heating systems for supplying potable heated water for utilization in homes and more particularly to a control system for turning on and shutting down the solar heating system in response to the solar energy available.
The reduced availability of conventional fuels and the resultant increase in their cost has brought forth a new look at the sun for supplying the necessary energy for heating of homes. The prior art is abound with solar collecting systems which generally had been recognized as cost effective only in geographical areas where the amount of solar radiation received was a maximum. Now, it is recognized that solar heat can be utilized almost anywhere to at least supply auxilary heat to a main system. The overall effieiency of the system does, however, depend on the available solar energy. In locations where seasonal or overnight outside temperatures are low, when the available solar energy level drops the efficiency of the solar system is nil. The energy in the system can be dissapated. When the outside temperature approaches or drops below the freezing point of water the water in the solar collector can freeze and destroy the system.
The prior art has recognized this problem. Closed systems have been proposed in which antifreeze is the working medium and transfers its heat to the water in a heat exchange. This entails expensive apparatus to ensure that there is no leakage of antifreeze into the potable water system. Other systems proposed use sophisticated costly electrical controls such as illustrated in U.S. Pat. Nos. 3,986,489 and 4,044,754. In the former a pump is controlled by the temperature differential between the water in the collector and the water in the tank to pump water only when the collector temperature is higher than that of the tank, but if the temperature of the water in the collector approaches freezing the pump must operate or use an auxillary electric heater in the collector. In the latter patent a sensor in the collector monitors the water temperature and controls solenoid valves electrically coupled thereto to drain the collector and shut off the supply thereto when the temperature approaches freezing. In these, as in any electrical system, the controls are only as good as the electrical power supplied thereto. In these times of frequent electrical outages and blackouts, a failure in the electrical power supplied to the controls will result in the destruction of the solar system on a subfreezing evening.